I didn't build the plastic extruder yet but everything else of my Tommelise is working already. The stepper motors are now powered by three TMC-222-SI driver chips which communicate over TWI/I2C with my ATMega 164p microcontroller. Using I2C driver chips, instead of the ones that I found in the old printers that I dissembled, saved me a lot of cabling plus they support microstepping.

For testing purpose I attached a milling machine as a tool head:

As an easy shape to start with I chose to mill a star.
For doing that, I first drew a star in gimp, reduced the color palette to black/white and saved it as a png image.

After that I wrote a small python script that looks for black pixels in the image and sends the microcontroller the command to mill a hole at the corresponding location.

It turned out quite well:

Of course it would have been nicer to create a real tool path instead of milling dots into the workpiece. But I will concentrate on building the plastic extruder now.